U.S. Pat. Nos. 4,473,842 and 4,549,206 describe types of apparatus for examining if a miniaturized electronic part is correctly mounted in a predetermined position on a printed circuit board using an image sensor to examine the printed circuit board. However, the apparatus in both of these U.S. patents can only be used with a circuit board having some degree of transparency since the illuminating means is located on one side of the board and the image sensor on the other side of the board. The images sensor provides an image which is compared with a stored reference pattern. However, one advantage of surface mount technology at present is the capability to use multi-layer boards. These multi-layer boards are not transparent. Therefore, the systems shown in U.S. Pat. Nos. 4,473,842 and 4,549,206 are not suitable for the present technology used in assembling surface mount components on printed circuit boards.
U.S. Pat. No. 4,677,473 describes a soldering inspection system for circuit components such as a component chips and the like which are soldered to a circuit board. This patent describes a system which utilizes a plurality of light sources and TV cameras to provide images of the solder connections from different angles and means to process the image information obtained. However, the system inspects the soldered connections after the components are mounted on and soldered to a printed circuit board and not the position of the components before they are firmly attached to the circuit boards.
Traditionally electronic components such as integrated circuits are mounted on printed circuit boards by inserting terminals of the components through holes in the printed circuit board. However, the electronics industry has recently been adapting to Surface Mount Technology wherein the components are devices mounted only on the surface of printed circuit boards. An automatic pick-and-place mechanism is used to place these type of components onto a printed circuit board and then the whole printed circuit board undergoes a reflow soldering process to solder the leads of the components firmly onto pads on the board. However, it is much more difficult to properly position the electronic components on a printed circuit board when terminals of the components are not inserted through openings in the board.
Surface mount components must be accurately placed on the printed circuit board since they typically have a large number of leads (18-84) and are often packed very close to each other on the board. Small misalignments of surface mount components can result in soldering defects such as bridging causing a short circuit or poor connections between the leads and pads. Other defects which can occur are caused by insufficient solder, excess solder, poor wetting etc. These soldering defects are not always detectable in electrical function tests since they can lead to failure after a printed circuit board is in service where it is subjected to heat or mechanical stress. For these reasons, manufacturers of printed circuit boards have very tight tolerances with regard to the positioning and alignment of surface mount components on the board. The automatic pick-and-place mechanisms can have accumulating errors in repeatability which, after some time, can lead to inaccurate placement of the components. Therefore, it is very important to inspect the placement of the surface mount components on the printed circuit board during the manufacturing process.
One method of determining the position of components on a printed board is described by M. G. Buffa in "Process Control For Automated Component Assembly of Surface Mounted Devices Utilizing a Machine Controller", Society of Manufacturing Engineers (SME), Vision 85 Conference Proceedings, March 1985, pages 5-180 to 5-200. In this method, the positions of the soldering pads are located on an empty printed circuit board and after the components are loaded onto the board, the positions of the components are compared to the previously obtained solder pad locations. However, the solder pads and component leads positions are not obtained at the same time and, as a result, the system is prone to inaccuracies which may be introduced by movement of the printed circuit board.
Another method of determining the position of components is described by N. S. Chang in "SMV - A Computer Vision Program For Loading Surface Mount Components", Proceedings of the SPIE, Vol. 557, 1986, pages 76 to 80. In this method, a vision system is used to determine the orientation and location of a component prior to placing it on a printed circuit board. No inspection is done after the component is loaded onto the board.